1. Field of the Invention
The present invention relates to the preparation of rare earth borides.
By the expression "rare earths" are intended the rare earth elements designated the lanthanides, having atomic numbers of from 57 to 71, as well as yttrium which has an atomic number of 39.
2. Description of the Prior Art
The rare earth borides are compounds which have desirable electrical properties. In particular, lanthanum hexaboride has excellent properties of thermionic emission and is used as an emissive cathode in high energy devices.
Presently, the industrial production of the rare earth borides is limited by the difficulty in preparing a product of satisfactory purity by a process that may be extrapolated to an industrial scale.
It is known to prepare lanthanum boride by reducing lanthanum sesquioxide by means of boron carbide or elemental boron, under reduced pressure [G. A. Meerson et al, Izv. Akad. Nauk. SSSR Neorg. Mater, 3, No. 5, pp. 802-806 (1967)]. However, the reaction is carried out at elevated temperatures; this not only mandates use of a graphite crucible, but also a graphite lined furnace must be used.
It has also been proposed, in U.S. Pat. No. 3,902,973, to prepare the rare earth borides by electrolysis of a source of the rare earths, in a molten salt medium containing a cryolite and an alkali metal borate. Such a process is relatively complex in light of the electrolysis temperature of 950.degree.to 1,050.degree. C., and it presents the usual problem of recovering the rare earth boride at the cathode, which entails a pollution problem due to the cryolyte.
The same problem of separation of the final product rare earth boride characterizes the process described in U.S. Pat. No. 4,260,525. This particular process includes mixing a rare earth carbonate, nitrate or oxide with boron, in the presence of aluminum, to dissolve the boron at a temperature of from 1,200.degree. to 1,600.degree. C., heating the reaction medium within such temperature range, cooling it, and then separating the rare earth boride from the aluminum oxide formed.